1. Field of the Invention
This invention relates generally to screen printing apparatus of the kind used to perform the printing process commonly referred to as silk screen printing. The invention relates more particularly to an improved mounting arrangement for mounting a silk screen frame on silk screen printing apparatus.
2. Prior Art
The printing process commonly known as silk screen printing is very well understood and extensively used for a great variety of printing applications. Accordingly, it is unneccessary to explain the process in any great detail in this disclosure. Suffice it to say that silk screen printing involves the use of a printing screen which has been processed to block selected holes in the screen in such a way that the open and closed screen holes form a pattern corresponding to the image to be printed. This screen is stretched edgewise across the normally lower side of a silk screen chase or stretch frame (referred to herein simply as a screen frame) to which at least two opposite edges of the screen are firmly secured to maintain the screen in a taut condition. During a printing operation, the lower side of the screen is placed in contact with a work surface to be printed, and a screen printing ink is spread across the upper side of the screen and forced through the open screen holes onto the work surface to form on the surface an image corresponding to the screen hole pattern.
The prior art is replete with a vast assortment of silk screen printing apparatus ranging from the very simple, such as a single manually positioned screen frame, to the relatively complex, such as automatic printing screen presses and multi-station screen printing machines. While they vary in complexity and overall design, most existing screen printing apparatus embody a screen frame carrier of some kind in which the screen frame is removably supported in such a way that the frame be quickly and easily removed and replaced. In a conventional silk screen press, for example, the screen frame is supported on a screen frame carrier which is rapidly driven back and forth to alternately ink the printing screen attached to the screen frame and then print a work surface. The present invention is particularly useful on such silk screen presses and for this reason will be described in the context of this use. It will become readily evident as the description proceeds, however, that the invention may be utilized on other types of screen printing apparatus.
The screen frame carrier of a conventional silk screen press comprises a carriage movably supported on the base of the press and a relatively heavy open rectangular master frame permanently mounted on the carriage. The screen frame is removably mounted in this master frame. Permanently secured to the master frame along its sides are means for releasably but firmly holding the screen frame in the master frame. In some silk screen presses, for example, the screen frame is releasably held in the master frame by clamps permanently mounted on the master frame.
This conventional screen frame mounting arrangement has certain disadvantages. Among the foremost of these disadvantages are the weight and high cost of the master frame and its integral screen frame clamps, and the inability of the mounting arrangement to accommodate certain types of screen frames, such as one having four rectangularly disposed screen anchor rollers joined end to end at the frame corners by corner brackets. The weight of the master frame is undesirable since it opposes rapid back and forth movement of the silk screen press carriage and thereby adversely affects the maximum press operating speed. The master frame weight also tends to increase the stress on and wear of the press parts and thereby increases the servicing periods, procedures, and costs neccessary to maintain the press in proper operating condition. The cost of the master frame, of course, adds to the overall cost of the press. Obviously, the foregoing factors are disadvantageous to some degree in all other types of silk screen printing devices which utilize screen frame mounting arrangements of the kind described. Accordingly, there is a need for an improved screen frame mounting arrangement for screen printing apparatus of the character described.